Signaling



Dec. 28, 19482 H. o. PETERSON SIGNALING Filed June e, 194e 4 Sheets-Sheet l Armen/Ex H. C. PETERSON Dec.'28, 1948.

SIGNALING 4 sheets-sheet v2 Filed June 8. 1946 mi@ d.

INVENTOR. /Zaza Pfff/@50M BY QQCUFQQ H. O. PETERSON SIGNALING Dec. 28, 1948.

4 shee'ts-sheet s Filed June' 8, 194e Dec. 28, 1948. y H. o. PETERsoN y 2,457,263-

, SIGNALING Filed June 8, 1946 4 sheets-sheet 4 xd; uw@

Patented Dec. 28, 1948 Harold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporationV of Delaware Application June 8, 1946, Serial No. 675,348

18 Claims.v (Cl. 178-66) The invention disclosed herein concerns sig-,i

nalling by means of electrical currents and is particularly concerned with receiving signals of the type known in the radio art as frequency shift telegraphy, frequency shift facsimile and similar signals.

In frequency shift systems signals are sent by' alternately keying on and olf currents of differ-` ent frequencies one of which represents mark and the other of which represents spaces betweenr mark signals. The keyed currents may be derived from a common source the frequency of which is shifted between two values by the keying energy. The currents may also be obtained by the use of two alternating current sources of different frequency, from which output is taken` alternatively in accordance with keying energy.

The keyed currents may be of high frequency or low frequency. In the latter case they may appear as tone modulation on energy of carrier wave frequency for radiation or line transmission.

At the receiver the frequency shifted currents are amplied and demodulated to recover currents of the mark and space frequencies or currents representing mark frequency and space frequency. The signals are a form of frequency modulation and at the receiver are subjected to a frequency selection or discriminating action to recreate pulse energy or direct -current potentials of varying magnitude which corresponds to the signals used at the transmitter for keying.

The'general broad object of my invention isk improved frequency shift telegraphy reception. i In signalling systems of this type if the mean frequency of the frequency shifted currents drifts the characteristic of the detector is no longer symmetrically related to the average or center frequency and the direct current potentials vary non-symmetrically with respect to a base value so that telegraph bias is introduced because the tube circuits coupling the same torecording apparatus are no longer excited as intended being direct current biased or Weighted in one or the other direction depending on the direction of drift of the center frequency. This reduces the effectiveness of the recording apparatus. In my U. S. application Serial #630,- 428, filed Nov. 23, 1945 I have shown an improved means for preventing such slow variation 4in the mean frequency of the currents from upsetting the proper action of the coupling tubes and recording apparatus actuated by the signals. This means involves the use of a relatively large condenser in a network having a long time constant between the demodulator output and coupling tubes controlling the recording apparatus. This condenser then responds to changes in magni.

tude of the applied direct current component as well as to changes in polarity thereof. At the. network output then an alternating current component is supplied which varies nearly symmetri-v cally about a base` value even when the detector output is biased olf, i. e. has a direct current bias caused by the carrier drift.

- An object of the present invention is an improved means for preventing this slow drift in the carrier frequency from introducing telegraph bias in the signal and hindering proper operation of the coupling tubes and recording apparatus.

To attain this object I provide in the coupling between the detector output and the recording apparatus a network of the nature described aboverfor reducing somewhat the effects of slow changes in the mean frequency of the signals. This means is similar in makeup and function to that disclosedin my aforementioned application and is used in combination with additional means which I provide in accordance with the present invention for limiting excursions of the alternating current component, at the network.

output, representing mark and space frequencies beyond selected values in either direction. In other words in the present improved system I eliminate the direct current component from the energy representing the demodulated signals and also clamp the resulting potential within fixed limits of variation in both directions of the signal excursions. When this is done exact centering of the signalk in the receiver is no longer necessary to provide potentials of reversingv polarity for controlling the coupling tubes between the detectorA output and the recording apparatus. Moreover if the signal wave before detection is sufficiently oif center the said potentials of reversible polarity may not vary symmetrically or as desired about a base value. An

object of the present invention is to provide means to recenter thesev potentials. i

Where the potentials are used to control a tone keyer tube, potentials which vary non-symmetrically with respect to a base value are desirable because the tube may be designed to operate best plifier. The bias-fixing means comprises a pair.

of uni-directional devices connected in opposed polarity across the coupling circuits and arranged in condenser charging and discharging paths including resistances across which the potentials varying as desired vyith respect to a base value are developed; 2

When my improved bias fixing means is'in the grid circuit of a coupling tube the said tube may draw grid current when its grid voltage swings positive and no grid current when its grid swingsv negative. The coupling tube discharge path then introduces a non-symmetricaieiectlin the couf pling circuits. The grid resista'iay lim'it'th'e grid current to a small value but an object of my invention is to correct entirelxt. this non-,s511311 metrical operation. To do this I provide an additional electron dischargeY device so arrangedt in the network as to draw-'a compensating current when the coupling tube is bias'dto eut-oit. The arrangement is schthat equal lleakage'paths are provided for positive and fnegative"strings of; thgrid potential f the coupling` tube. Then the discharge pathsl have" no eifect on the bias centering. l'' "i i The network feeding the control potentials to the"'g'rid of the tonefj lrieye'r tube" includes a r'capacitor. When` signals arecoziiirig"iny a negative bias devel'o'peddf'slet up' on 'the control grid ofthe cpnti'oi tube. thei endof signalling capacitor inthemgid'- circuit ofthe controll tube may 'sha'ge-fpen'iitting"asoinetvhat less nega# tive'A voltage t6 bfuildgip tlj *grid ofthe'control tube.' 'f-Ilhdistirbajces an'd""' r4e"s-ii;iftl iesyst: :mettesse areas' appareils 'A f rthbbjetofny inventi o providelnieans este' iff-tte essere aos'c'qneoljoe can' time whit-netas ce i tiene assistera mais' 1in' ses,

Ve'. "tigris 3.15.@ @automaat @engagement of 1 land 2Uand includes 'iriladdiwonaltube (lo g t, fili b ample as shown in said patents.

tems as disclosed in Schock U. S. application Serial #632,978 led December 5, M945. In these embodiments my signal centering clamping and so forth network is connected to the output electrodes of the gating stages. The gating stages are excited at their inputs by detected frequency shifted signals.

y `In Figure 1 rectangle I0 represents a current 4amplitude amplifier and limiter operating at an appropriate frequency, to amplify and limit frequency shifted currents the frequency shifts of *'Whichare in accordance with telegraphy, facsimile or similar signals. The limiter I0 might for example operateat 50 k. c. or higher or lower 'or even as low as a tone frequency shifted in agcdrdance with the signal. The limiter I0 supplies output which is impressed on equipment in I2 including a frequency discriminator. In this discriminator frequency shifts of the tone or higher frequency current are converted into cor-A responding frequency and amplitude shifts. This frequency discriminator may be of the olf-tune type as inConradfs U. S. Patent #2,057,640 or modifications thereof or as disclosed in the Seeley U'. S. Patent #2,121,103 or modifications thereof. The detector in lli-coupled With the discriminator output may be of any approved type such for ex-v The detected signals at the output mayy vary above and below substantially Zero potential and these Variationsv may be symmetrical whenv the received signal as applied from unit I0 to the apparatus in unit I2 isV centered on the characteristic of the frequency discriminator in I2'. These potential variations appear across the resistor RI andv are supplied byV coupling condener C and series resistor I5to the control grid Il of an electron discharge device Y I8.

The tube 'I3l may' be a coupling tube supplyingl output to recording apparatus such as a teletype- Writer or tp an outgoing line. In the embodiment discussed tube I8, i's-a tone keyer and itsv anode circuit includes a resistance 2l)Af in the grid biasing circuit ofthe tone amplifier tubes 22 and 24. A tone source 26 is coupledv to the primarywinding off'a transformer the secondary winding of whichis connected in push-pull relation between the control grids of '22 and 2li. The potentiometer resistor PR."across a ydirect current source notshovvn supplies operating potentials for the tube I-Sgandthe tubes 22 andA 24. The tone keyer arrangement-is such that when tube Ilis conductive theipotential drop in the! resistorZQ biases the tubes 22-and 2 5 to cut oft thereby blockingenergyA off tone frequency suppliedby sourcel 26 from-being applied: to the output-lines 30.- When tube- I8 iscutf'offthe potential at the anode end of resisto'r 20AY rises that is becomes more positive toI turn on the tubes 22u and 241to permit energy ofr` tone frequency from source 2 5 to be fedto thev cn'itgoingl lines '30. lThis might be the mark condition 1n the'y system. A- sirnilar tone keyer is` show-n andlolaimed'in-my'U. S. Patent-#2,23'7,63l

dated Aprile, wir.

lThe condenser -C is relatively large and with resistorRI, which may bein thediscriminator detector output, operates asdescribed in my U; Si application-Serial #630,428l filed November 28,

1945l to supply tothe control grid I1; potentials theA polarity,Y of#y which reverses with the keying* evenfthough the incoming wave is miscentered: with respect to thediscriminator characteristic.' Unilateraldevices in they forni of diodes 32 andy fare connected asi shown in opposed polarity betweenY the inputr terminal ofV resistor I5y and-4 ground. ySources 33 and 35`ar`e -in these connections.` LAs-the'signal'potential swings back and forth a directcurrent component is developed in the resistor RI but cannot be applied through con'denser C to the grid I'I. lsuperimposed on this direct current voltageis afcomplexV alternating `currentvoltagerepresentingthe facsimile or telegraphy signal. As long as the incoming signal is centered the variations may be ysymmetidcally related to a base value if desired and only the al# *ternating components are ytransmitted tothe grid I'I. However if the center frequency of the incoming wave drifts there is a slow change in the direct current component across RI and this voltage may be transmitted to some extent but is severely attenuated-by'condenser C.- The tube vIt as a consequence is nolonger subject to a direct current bias and its operation is not affected vmaterially by slow changes in the detector output. 'I'he function of resistor I5 is to limit the grid current `drawn by thetub'e I8 to a small value when the potential swings positive. Resistor I5 may be of. the order of two megohms in a typical circuit while condenser-C may be of the order of one mfd. and resistor RI `may be of the order vof 100,000 ohms.

` The apparatus and operation of the bias clamping tubes 32 and 34 will now be described.V In this description reference'will be made to Figure 4 of the drawings wherein is shown a characteristic typical of this'tube and tone keying circuit. As increasing negative voltage is applied to the control grid. I'I, the tone keyer including tubes 22 -and- 24 begins `toproduce tone output at about minus one-half volt and-comes to full output at about minus one and one-half Volts. The tone output-characteristic of these two tubes is shown by curve A in Figure 4. The voltage applied to the control grid I'I of the tone keyer tube I8 is shown by curve B. The output of the tone keyer remains constant for all values of signal potential applied to the grid I'I which exceed one and onehalf volts negative and theoutput of the tone keyeris zero for all values of voltagefapplied to the control grid I'I from about one-'half volt negative on up into the positive range ofvoltages. In a typical embodiment the voltage variation across resistor RI in going from mark to space might be on the order of twenty volts. In 'that case I would like the bias fixing tubes 32 and 34 to function in such a manner that the control voltage applied to thegrid I'I of tube I8 in the tone keyer swings from minus 11 volts ori-mark to plus 9 volts on space. This causes the tone keyer output. to change from full on to full off at a point midway between mark and space voltages, which is desirable, to avoid telegraph bias when the signal shape is not the idealized square wave vshown in Figure 4. In other words since the tone'keyer I8 turns, on and off for biasing potentialsy as xed by the bias clamping tubes 32 and 34 well within the peaks of the signal or control potential excursions mutilations therein or rounding Off of thecorners thereof are unable toaifect control of the tone keyer and as a consequence introduce telegraphybias inthe tone output.

` To accomplish this desired positioning of the Voltage wave on the control grid I1 the bias `of clamp diode 32 is in the example given about'9 volts positive whileithe bias of clamp tube 34 is about 11 `volts negative. The cathode of the "tube 32 is for example made positivewith respect to ground about 9 volts `by source 33 while the anode of. tube 34 ismade negativeI with respect to groundv about 11-Volts by source 35. .Thus in vthe example' given the potentials on the right hand end of condenser C must swing more than 9 Volts on thepositive excursions to make tube 32 conductive' and more than 11 volts on thenegative excursions to make tube .34 conductive.l Y

Now if the signal voltage wave isnotcentered as desired one or the other of the two clamp diodes 32 and 34 will draw current which will placeA a charge on condenser Cin theproper polarity and of sufiicient magnitude to cause the wave at the tone keyer control grid 'I'I to be properly centeredt-ll v. on mark; A|9 v. on space). For linstance assume that the frequency of the input' at-I0 has drifted so that the signal wave at resistor RI swings from minus 5 volts for mark to plus 15 volts for space. When the signal is 4on spacey with plus 15 volts, the unidirectional conducting device 32 draws current through resistor RI to place a negative voltage ,charge across con# denser C. Thisr negative voltage charge will amount to about 6 volts (l5-9). The voltage impressed on the input of resistor I5 feeding the control grid I'I will be the algebraic sum of the voltage across' resistor RI and the charge voltage across condenser C. In this example then on space a voltage equal to plus l5 plus minus 6 or vplus 9 volts will be fed to the input of yresistor'l5. On the negative excursions of the signal to'record mark the voltage/applied to the input end ofresistor I5 is equal to minus 5 volts plus minus 6 volts or minus 11 volts on mark. Thesame kind of action takes place if the frequency drifts in the other direction so that there is an excess of negative bias. In that case diode 34 draws current Iwhichplaces a positive charge on the condenser C ofza magnitude which will result in proper tone keyer operation.

Now returning to the example given above, wherein the mis-centered wave results in swings of +15 lon space to 5.on mark and a negative bias of -6 volts is built up across condenser C and a space voltage of +9 is applied to the resistor I5, .if due to noise or other effects this 9 volts is exceeded materially diode 32 will conduct and square off or limit the wave. In somewhat like manner if on mark a voltage in excess of -11 volts is applied to resistor I5 diode 34 will conduct to limit and square off the mark voltages.;

In the embodiment of Figure 1 when the grid I'I swings positive grid current will flow `but this grid current is limited to a small value by resistor I5. However the small grid current does introduce dissymmetry in the circuit because when the grid I'I swings negative the tu-be is cut off and no grid current flows. Obviously it is preferable to have equal leakage or biasing circuit current ilo-w on `both excursions of the sig# nal being recorded so that the discharging. or the leakage paths haveequal current flow theres in on positive and negative swingsv and these discharge paths `have no effect on the bias centering. In Figure 2 I provide similar leakage-paths for both swings of marking potentials. The rst path includes resistor I5 as inv Figure 1. An additional pathincluding tube I 8 with resistor I5! is provided. When thepotential on control gridI'I swings negative the cathode of tube I 3 also swings negative to make this tube conduc tive at nearly the same instant tube I8 becomes non-conductive. When the control grid ofvtube -I'I is swung positive grid current flows in the resistor I5 of an amount substantially equal to that flowing in the resistor I5 lwhen-tube I8 is conductive. On the positive swing'stube I8' is 'cut off sothat no current flows in resistor I5".

Ioyovlthexlsolraige. eathsfarefequal ombothfswings xiixhessignabvoltage.- c fr in ithemembodiment :oiigures s1 tand; 2 when vsignallngxhasfstopped the; charge .fon f condenser C will leak-sofffslowlyesocthat after afiew.-seconds stletone i-ikeyer :control .gr-id i [l1 *voltage* will the mearmerozpotential andesmall negative excursions idue :to :atmospheric .-noise Vfiwill '1 cause the w tone 'keyemtoobe non-:conductivezto:increaseaits anode qitential end causezthe .tubesiffa'nddatotransmitlbursts :of #stone `totthe :output zlines. :These Jnurstsl of i tone-:due vLto incise during i idle f .periods mill esome iin'stances zprove fdisturbing .to the radio nperator. InrligureSItfshow'means which 'seme "-.to :block: oi tor .fsquelch the operaton :of .lthetonekeyer fas i long moise '.peaks are vdoelowfafsettthresholti'vrilue. '.This -Lset valuesismf course, smaller than `the .'xmrntalsslgnal -r-izalue. This @means @includes :Sanveleotronicontrol 'f device having :its vcathode fconneetedato :thefinputnend #ff-resistor'l andfitsianode cireui-tr'coupled'totthe iuontrolfsgrid '3 9 roi f an'leliiditional tubeifl 0 isupplieu withipotentialfromrthercommon' sourcerandtfrom a; ysourcehldllroldiug'the:'oat'luode 'ofatuhe all Omegativewithrespeettoiground. mhezgridastisoon- "nected to the cathode :by a tbiasing resistor :50

`spatcietto minus Illlfvolts fommark. fAlso assume that the bias source llfholdsitheioathodefofituhe Asevenf (7 )woltsnegativewith respeclito ground. Ehen iwl'aen Ltlre signalling :potential at resistor eltswings negative x10 volts on marhfcotrdenser 33,1 roharges ,to zbout lminus xvolts '.thereby nbitts- .ingitubezto :cutioff iand hence .tube C fhasmo effect :onitlre ,-operationoofathe toneikeyer during normal flsignalling. In athis respect mote lzthat when athemotential :et :resistor M15' swings `in fthe positive :direction the :ztube 36 .is noneoonduotive. Resistor sxoi such na, value as toidisoharge condenser 31 very .slowlm fisayion =.;the `:order :of

`seconds. rIf;v signallingA stopsl .therenwilL nmlorieer .be megativeswings on ,the cethodeiofadiode 36 A and :condenser 1 gradually i discharges -allowing :time aura-w1 current '.through mesistor 'Ril =1to make :more rnegative the ...potential :at theanode :end :thereof .to Akeep :tubes 22 zand i2# blocked ,'oi

.as 1they1wou1'dbe inathelspaee .condition inttheembodiments.described. hereinbeioreand illustrated inzFigures i1, mande '.Irhaveeshown my improved 4 biascentering and loies-.;clurr11:in g7 and .tone keyer grid vcurrent acompensating, i and noise suuelchixtg .oircuit vas being I.used .with .single ire- `weiters undy conventional-1deteetorisystems. AItuis equallyadvantageous tozuselmylimprovredfscircuit .arrangementsiin diversity; systemsnf: allatypes and insparticular sin Adiversity.- system's of lthe docking circuit-.controlled gatlngztubettype. {For example, .myflimprovedihiasentering-zcircuitsSandfso:forth mayfbe connected .to :the foutput fof ifthe 5 gating .tubes of `'afsystem as .disclosed'in :Schoekeetfz-al. U. fiapplication :Serial #1632;978,4 filed December Then the signal f'centering, fand bias clamping, Jand Lcompensatingfvandnsquelching:circuits maytsupply a tone keyerlasin @Figures `l, S2,

3, e-andfi @relooking circuitstagesintturnsupplying te, ltone f'keyer or lteletypewriter as iin -fsaid Sehoek etalsapplication. rIn` the embodimentstof Figures. zand 6,-aeference numerelsorr-thgatln tubesfand ,theirecontrol-teircuitsgzeorrespondingtto thoseausediinthesaid'afDPlioa/tionfareused insofar astpossibe. -#Theseveralfsignel Yersionsare appliedthroughwcondensers `'|.le.nd-Il'ftd-thementrolsgrids -remdfl lof-.gatingtubesefanddf. Whieheverttgating etube nis' Qpen-fampIiesthe--deteotedxsig-nalfand:thetsamefappears ascrose aff-commonfloaldtimpedance. -fIn:Eigure this loadiim- `patienceut-lso.:servesthe functionf of the resistor Rl r in FrFig. 211. The xresistorfwRl condenser`` C land diodes a32vand-34fsewe to 4I center andi limit tand .clamp .fanso-vforth the;.keyedmotentialsapplied .to-the @control grid:54of;.thecouplingtubediUsubstantiallyrinithewsame :mannerpastthese functions are:eeomplishedv bym! ,1 Cffarrdfso.y forth-.tin Figure 11 fof lthe -zdrawin-gs.- .The ftube- 50 .operates :as sa. cathode =i`dl=lower :stage 'with resistance -5'l:-a.s fits .output impedance. "The :coupling tube 611 supplies ltherkeyeclisigi'ials tlfuollgh'af-low;passflltering net` -work Tuite-1, the controLgridtl sof the. tonefkeyer .'tubewl.

:Glamp tubefzekeeps-.grid 54:.from-.;goingposi tive. vHenoe grdillggoesifromf..tofsomevnegative value of Voltage V:This '-negatiue limit .lseestablished by clamp tube 34 and biasvoltage, srFor Omolts ongrdt 54, ltlieiplate eurrentlthruresistor 51 :ggenerates en @voltage 'approximately sequel] .to voltafgezofibiasaiiso theresultantfmroltageappliel ftoiinput'mfrresistor :58 z is fapproximately:."zero, :for

which-scendition @tube rf8"zdratwsrentrent,` andino tone Yistransmitted:ahy ztheotorretkeyer.

When gridfidv-goesenegative, the platedcurrent .thxurresistor :i5-1 decreasesmaking :the .'I. R. drop :in .resistori'xl vless :than :fthe fbiasli iso :that :the resultant is eai'negative'voltage. applied :torreslstor 58, ".which-in'lturnfreauses 'cut-'ioiiofz-vplate current .in f tubeffw, ewhieb: causes .tone :output to beitransmitted.

:When there is :no ikeying" present, *dane charge nnfoapaoitol-:C lee-ksfoff :thrul'rightresistancer softhatfthe-voltage onigri'drliisfthe negative Noltegel of hiess 55. fillhisrresults inra' melaztivelyfsmall fvalue'fof;plateieurrentrthruiresstor'51u50 thatthe resultant:ispeffnegativef'voltage ppliedltomeslstor .58, which .cuts `oifzplate .':wrrentfin ftube .1:8 yand .causes :tone tothe"z transmittedzby tlreztoneskeyer,

fIn zthe preferred fem'hodiment Cof Figure .6 fthe emoties of atheflgating tubes fareccoupled ttorraommonsloaddmpedance rZ'lzSears lin :said Schocksetlrl. application, One iof Ithe gating atuheg4 is :turned .onlbyetheeontrolpotentieleapplied-:tolitsirstfc'ontroltgrid 264ml #Millt .-,sothatzthev detected-.signals appear morose-resistor 215 an'dfarefed by foondenserft itc .the potentiometer :resistor 388. .rA coupling daube 60 in Vthis embodiment precedes .therlompassllterlm. v\.'1hefcoupling ;tube1:60'.isf.uf .tlrecathedeltollower typerso thatiamplieiorire- .zeyed fsignalipotentiels rane 'fed toilow pass 11.0zandeagpeareaerossiresistorRl '.whichzis ofthe load circuit fof .tube @69 4und :also "with capacitor C :clamp tubes d 2 :and :34 :serves the .purpose described -i-.hereinbefore .The :signal fvariation's at :RI olowithe ssignal variatonsiaztt T.the @ridi'f .tuhedllfandwthetcouplingttube60l inithisfembodiment with tubes :32;and dfa'nd resistors:53,i51 and w58 -functionfes described.' abovei in conneetlon witniigure '15 togenertttei iniresistorflfa :voltage about :equal ttoI the Voltage' 55 When thepotential .onthefgrid-.Sllzis zeroso thai-then theivoltagefat .thefinputendntresistor 58 isiaboutizero and tube .i8 rlrawsecurrent. AWhere thefgridfd :swingsrnegwative (mark) r.theeuropthrough.571 decreases und -thexbias 55 predominatesxto cut-fotti plteleurrent .inftube dwandesendout.tone-renergy. Whenmere isno signal applied to the tube B0 the charge on capacitor C` leaks oli through high resistance 53 and the bias 'on'.grid'll; rises toa negative value set by bias source 55. The plate current through 1 is reduced and a. negative voltage is applied tothe resistor 58 tocut-off plate current in tube I8 to send out tone current. a y v l i vWhat is claimed is:

l. In a, signalling system in combination, an impeder across which a signal potential appears which potential variesA inmagnitude between maximum and minimum values separated by a substantially iixedamount, said potential being of an averagevalue-which may vary slowly to thereby change said maximum and minimum values, an electron discharge device having a con-A trol grid coupled to said impeder to be potential modulated'inaccordance with the signal potentialthrough a selected substantially fixedy range of substantially fixed average value meansin-the said coupling for preventing changes in the average value of said signal potential from materially changingv the average value of said potential modulation of said-grid `and means in said coupling for preventing variations in the magnitude of said potential modulation from exceeding selected maximum and minimum values,

2. A system as recited'in claim lvwherein said rstmeans includes saidmpeder and a relatively large capacitor between said impeder and the control grid.

3. A system as recited 'in claim-Al wherein said device includes v`a cathode and wherein Isaid last named means includes diodes lshunting the grid to cathode impedance orsaid device in opposed polarity. y l

, -4. In asignalling system -`in combination, an impedance'across which a* signal: potential appears which potential varies in magnitude between maximum and minimum values separated by a substantially `ixed. amount, said potential being of an average value which may vary slowlyl tothereby change said maximum and minimum values, an electron discharge device having a control grid coupled to said impedance to be potential modulated in accordance with the signal potential through a selected range above and below a selected base value, means in said coupling for adjusting said base value, to thereby also adjust thelimits of. saidpotential modulation yrange,

means in said coupling for preventing changes in the average value of said signal potential from changing the base value of saidpotential modulation of said grid 'and'means in said'coupling for preventing variations in the magnitude of said potential modulation from exceeding selectedy maximum and minimum values.

' 5. Asystem as recited in, claim 4 wherein said device includes a cathode and whereinvsaid second; and third means includes diodes in opposed polarity in' -shunt to the grid tocathode impedance pling between one end of said impedance and the cathode of=said device, a relativelylarge capacitoricoupling the other end ofv said impedance to said-control grid, va discharging, circuit Afor said capacitor including resistance4 of-a value such that the product or the capacitance of.the"capacitorand said resistance is large as compared to the rate of variation. of said rst named potential, saidfresistance including a diode coupled in shunt to said control grid and cathode and means for limiting variations inv saidpotential modulation between two-substantially fixed values including said first diode anda seconddiode coupled in opposedpolarity'betweenthe control grid and cathode of said-device.

7. In a receiving system for. frequency shift telegraphyand. facsimile in combination, a resistor wherein signa-Isl having an alternating current component and a direct `current component appeansaidalternating current component being of a magnitudewhichchanges in accordance with signals about Ia base value, and means-for eliminating said direct current component and establishing said basevalue as desired including output terminals, a relatively large capacitor coupling the high potential end of said resistor to one output terminal, a connection between the other end of,;said-resistor and the other output terminal, va'iirst` diode having its anode coupled to said one output terminal and its cathode connected to said othenoutput terminal, a second diode having its cathode coupled to said one output terminal and its anodecoupled to said other output terminaland. means for applying'an adjustable voltage to at least one diode for biasing the same as desired.,

8. In a signalling system, a sourceof potential which changes betweentwovalues in accordance with signals, a tube having a biased control grid and having a cathode and having output electrodes coupled` to signal reproducing apparatus,

in the grid current of said device with variations4 in the magnitude of the potential modulation.

10. Asystem as recited in claim 1 including meansfor compensatingior the elTect of changes in the grid current of [said device'with variations inthe magnitude of the potential modulation including, a second electron discharge device having its cathode coupled to the control grid of -said first device and its control grid coupled to the cathode of said rst device.

11. vA system as recited in claim` 8` including means for rendering saidrst tube ineiective to actuate said signal' reproducing apparatus in the absence lof vimpre'ssed'signalpotentials for a selected'timeperiod. "i r 12.. In a system-for recording signal potentials i' which vary lr'n'etvveeri'-two values representing difmeans .when said.fpotential?modulation is inter-iA mal-:mees

rupted` for: a l selected-i. timesl periods including: af diode having` yits :catnodeyX coupled'todthee control grid: ois' said device iandzits- :anodeacoupledebyra capacitorf` and a. negativesk sourcerofpotential-toa thecathodeof'isaiddevicea arnesistor iwistmntatu said cafpacitorf. andi assecond electron: discharge'A device-havingvf its.I controligrid f and',1 cathodeeinf shuntitozsaid :lastnamedresistorrandits output electrodes in parallel witiiithef.-outputelctrodes offsaidrstfdevices.

13: Inl a.systemiforxrecordmgsignalipoteutid: wiiiclr'vary. between two ivaluesirepresentigtfdflfferentzsignalling:conditions; said potentialsobeing. of an averagel valueewliicl: mazavarytfslowlwtoi thereby.changersaidtwofdiffrentfvales; aix-ielecel tron discharge device e having?" outjniti` electrodes andi having a: controlgridr' coupledz. to emitir-pedance to` be` fpotentia'lt modulated in# accordance with the signal potentials'rthrough azselctedffsulie stantiall'xT Xedfrange-lof sulistantiailwxedi avere age value; means-1in 'said couplingef or -preventingf changes in the averageavalue of said* signal-ipo@ tentia-l; from?,A materially; cliangingtlie average. vaine :of' said potentialfmodlatinsoffsaid-igirid signairepnoducingfmeanswoupld tohsaidi output electrodes;` and'meazisaforsirendrirgesaididvice; iinmeativet tm controlaI said signal' reproducing means:V inf.. the` absence of?? potential' mcdulatonf inclndingza `diode having-fitsscatliodecoupleetto. the control grid of said devices and itsL anodel coupledibvfanrcapacitorfana niegatiie source-of potential to thercatlodofsaidffdevice-fatresistorr in shunt :tof/said.icapacitorfandafsecond elctrorr dischargeidevi'ce iliavingdts "contolf gridiI and-cattiodef-imsldunt:to'isaidi1astnamed-3resiston andfittI outputi. electrodese in paraile triei ctutputi electrodes of saidfst device.i

`-lli mia-system .forrecordi'rgqs'ignal potentialse which*varylloetvveei'if-tunmfalzes:representlnemiifL ferentisianalfling conditions;saidipotentiailsebeingJ off any aeragei vaiueewlfiih \mayrvaryu slowly# t@ tlierele'y change said tWodif'rentiva-l-iies arr-SelectL tron Ldischarge= device havizigfja/ controfgridlcoue pledito saidfimpedance to@beepotentialm'oclfuitv.ted im accordancewitii etheesignailpctntialsvthrough' a selected substantialitfixed rangwofdsubstme tiallyiiiiied average value;-meansiinisaidkhoupling fom preventing; changes!` ini tifo.s averages valiie= fof? said sigifiall` potential fom n'iateriiilyl44 clangfng;

the ais/"oragex value oil-1isaidpotentialmodlatiom ofiisaidrrgrid, means in saidflicouplingrfoizfpreventingdiariations-:inx the: magnitudefoids aid 'potential modulation( fromw4 exceeding selctedirmaximumfand?" mninnim:A values; meansvi:l compensatngi ferr: changea; in i tngxzids ouvrent imsaid-:i devicex caused:by:changesiimthefmagnitude ofosaid por tentiai;modulationcandimeanseforerendexng:said device inoperatveett control saidi'signal reproaducingcmeans:inLthezabsenceeopotential modulation:W

15.` Imasignallingzsystem::cssource-:of 'signals-fl which anamnese hetweem ahondarY zero vpotential?. which represents one signallingif conditioner,v and?! aanega'tiye :potentiawhich rrcplsentsl .aflsecond signalling conditionze a? controle tube-shaving): a: controlsV aridi: and: cathndeand Having; output electrodes arram;edztogoperatasignalireproducing.r

rentcomponentand ai direct component-:appear said i5 ait'ernating current component-being; of amagnitude wlich changes in accordance# with" signals about af base9 value` which A may 1 vary-' in an undsiredfmanner; and." means forv eliminati ing-fthe efiect'fof variationsof 'said basef lvalueandfior establishli1g:u said tasef value asv desired' includingf an electronh discharge couplingA tube having" a control" gridif coupled' to said resistor, saidtube having; output electrodes including-a cathode; a;- control tube having?outputelectrodese coupld tov a1' utilizationy circuiti4 and havingIk ai controle! gride'and"l cathode; aglterf coupling': theZ cathode" offsaidf'rsti tub'e -tothefcontroligrid of" said secondVl tube, saidffilternincluding-arrf irri-- apparatus@I meanssfor imnressingzsaidpotential:

pedance i in l thel cathode:v return* circuit'n ofi said iiistftuicfe-y and f a -f rela-tivelylargeiseries capacitorx and sluntings-diodes-of opposed polarity-Min said" r-st or third coupling.

18. In aw signal -f recordingt system inA combina: tion, af-resistorf=Whereinsignals havingran alternl natng'currentf component and-a direct currentL component aboutfwhich l*said*alternatingcurrent' component alternates in accordance Withdiir-i- -ent signalling` conditions; appear; means" for,

eliminatingV said"Av direct current componentand`l establishingra new base valuevarounwwhichmn alternating currentk i' component` representingrthc. signals'- varies" asYH desired including an electron. disci-:large deviceh'aving voutput "electrodesgincludi in'ga-load impedance coupledtoutili'zaticrr means; andrto asource of `direct"current`potentiai', Vafi-.wrt conductor circuitil includingrin one lead fa' rela;

tively: large capacitorA and iresistor lin series con-"- nected i from spaced--l points i ontsaidiist resistor? totheicontrol girdiandcathod'e'offsaid tube; two diodes connectedn in" oppyosedi".polariti,Y througli':

i potentiaisourcesof opppsedpolaritvinrshuntt.;

saidjtwo conductors, .asecondielectron' dischargeV device* having output?l electrodesy includingj an anode coupled" tzr-said source off direct i correnti through said" load impedanceI and* inclfidir'ig' al cathode connected by1 a` source' of'ipotentiai, to; tlie-7 cathode-otsaidliirstr devce,`,a diode andfa. capacity inA series t'connectingj the high potential side of "said two conductorfcircuitto the cathode of* said' second'. electron' discharge device' andi' a' resistance'ain' shuntto'the control gridandcathodeiimncdan'ce;ofsaiwsecond 'device' v .1 Of' PE'1`E1R7SD1\I`N REFERENCES CITEDI *T l'ie'ifollowing references? areaofi?recordfiin; the# flier- 015' thisfpatenta i UNIFIED', STATES RA-'I'ENTS'i A 

